A Beam Division Multiple Access (BDMA) system is a system for providing a spatial multiplexing gain in such a manner that a spatial selectivity is provided by forming a directional beam other than the existing omni-directional beam between base stations (BSs) or between a BS and a user equipment (UE),
Regarding the spatial selectivity which utilizes the directional beam, one of important issues is a half-power beam width which corresponds to an angle at which an antenna gain is decreased by half against a maximum antenna gain, and is closely related to the number of array antennas.
When the directional beam is utilized in wireless communication, an antenna gain varies depending on a location of a transmitter/receiver, which is directly related to a Signal to Noise Ratio (SNR). That is, the transmitter/receiver may be spatially located within a specific range (in general, a half-power beam width) in order to satisfy the SNR to maintain communication.
Accordingly, a beam-forming technique in which the transmitter/receiver can form a beam in a mutually desired direction is required in the BDMA system which may utilize a directional beam and perform communication (from a BS to a UE or from one BS to another BS) between various transmitters/receivers located in unspecified locations.
An Array Factor (AF) which represents a spatial size distribution of a beam is a function of a delay size of a signal flowing through an antenna and an incident direction of a received signal. Therefore, the beam can be formed in a desired direction by regulating a delay of the signal. An element for performing such a function is a phase shifter.
If the phase shifter is an element for determining a direction of the beam, a factor of determining a beam shape (i.e., null, beam width, and the like) is a size of a signal which flows through each antenna. The size of the signal is regulated by using a Variable Gain Amplifier (VGA).
For example, since a size distribution of a signal can include a binomial distribution by using the VGA, it is possible to form a beam that does not include a side lobe, that is, a beam which is not radiated in a direction other than that of a main beam among directional horizontal patterns of an antenna.
However, in a normal case, due to non-ideal performance of the phase shifter, the VGA takes a role of correcting a size difference of the signal which flows through each antenna.
Accordingly, there is a need to utilize the array antenna, the phase shifter, and the VGA in order to maintain communication between fixed/mobile transmitters/receivers which include a spatial selectivity in the BDMA system.
The biggest problem occurring when the aforementioned beam-forming technique is applied to meet the purpose of the BDMA system lies in that a complexity of the system is significantly increased to form multiple beams.
The BDMA system utilizes multiple beams to increase a channel capacity by using a spatial selectivity. In order to generate and operate the multiple beams, a beam-forming system including an array antenna corresponding to each beam is required.
As described above, the beam-forming system requires a phase shifter for regulating a direction of a beam, a VGA for compensating for a gain (or loss) error of the phase shifter, and a power combiner/distributor for combining/distributing a plurality of signals. In addition, in order to reliably operate a plurality of circuits within a signal path, a circuit for monitoring and correcting the operation is additionally required.
As a result, a system complexity is increased, which causes a problem of increasing a system cost and increasing a system error rate. Accordingly, for the BDMA system, there is a need to develop a technique which can perform beam-forming with a much simpler structure.